Method for shielding printed circuit board circuits

ABSTRACT

A method for shielding one or more circuits ( 21, 21′ ) of a printed circuit board includes depositing a layer of dielectric material ( 43 ) over a printed circuit board substrate ( 22 ) and the printed circuits ( 21, 21′ ), creating a trench-like opening ( 44 ) in the dielectric layer ( 43 ) such that the trench-like opening ( 44 ) surrounds the one or more circuits ( 21, 21′ ) to be shielded, depositing a layer of metal ( 27 ) over the layer of dielectric material ( 43 ) and within the trench-like openings ( 44 ), creating a solder pad ( 24 ) at each location where an electrical connection is to be made to the printed circuits ( 21, 21′ ) by removing a border of the metal layer ( 27 ) surrounding each connection location, and providing a microvia ( 25 ) through each solder pad ( 24 ) penetrating the dielectric layer ( 43 ) and terminating at the metal of the printed circuit ( 21, 21′ ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. application Ser. No.10/263,006 filed Oct. 2, 2002, now U.S. Pat. No. 6,990,734 which is adivisional of Ser. No. 09/775,460 filed Feb. 5, 2001 (now U.S. Pat. No.6,515,222 issued Feb. 4, 2003), and assigned to Motorola, Inc., fromwhich priority is hereby claimed under 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the field of shielding printedcircuit board circuits and in particular to methods and apparatus forshielding circuits on printed circuit boards utilizing high-densityinterconnect technology.

2. Description of the Prior Art

Printed circuit boards can contain many separate circuits where eachcircuit is generally arranged at a separate location on the board.Inasmuch as some circuits emit radiation, shields have been employed toprevent the radiation from reaching the environment outside of thecircuit board. Typically, in the prior art, the circuit shields compriseformed metal cans or metalized plastic cans which encompass the circuitwhen mounted to the circuit board.

The prior art metal or metalized shielding cans necessarily utilizespace on the circuit board. Circuit board space is a valuable commodityin today's technology. Smaller circuit boards and/or additionalcircuitry can result by increasing the useful space on a circuit board.Thus, what is needed are methods and apparatus for shielding circuits ona printed circuit board which reduce or minimize the space used by theshielding on the circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, advantages, and features of the invention willbecome apparent to those skilled in the art from the followingdiscussion taken in conjunction with the following drawings, in which:

FIG. 1 schematically illustrates a typical printed circuit board whichincorporates the present invention;

FIG. 2 schematically illustrates a portion of a printed circuit of thecircuit board of FIG. 1;

FIG. 3 schematically illustrates a cross-sectional view of oneembodiment of the present invention;

FIG. 4 schematically illustrates a solder pad portion of the uppersurface of the printed circuit board of FIG. 1;

FIG. 5 schematically illustrates a cross-sectional view of anotherembodiment of the present invention;

FIG. 6 schematically illustrates a cross-sectional view of yet anotherembodiment of the present invention; and,

FIG. 7 schematically illustrates a cross-sectional view of even afurther embodiment of the embodiment of FIG. 6 according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure. Further, the terms and phrases usedherein are not intended to be limiting; but rather, to provide anunderstandable description of the invention.

Reference is made to the drawings, wherein like characteristics andfeatures of the present invention shown in the various figures aredesignated by the same reference numerals.

Reference is now made to FIG. 1, which schematically illustrates, incross section, a typical printed circuit board 10 which incorporates oneembodiment 20 of the present invention. Circuit board 10, for example,can comprise one of several circuit boards or an entire circuit board ofan electronic device such as a cell phone, a pager, a two-way radio, orother like analog or digital device. Circuit board 10 thus includes aplurality of circuits, connections between circuits, transmission lines,transistors, speakers, controls, microphones, and other like circuits,electrical apparatus, and components 11. Generally, a group ofelectrical circuits are arranged side-by-side to form a functionalcircuit block which generally performs a specific function within theoverall circuitry of the printed circuit board 10.

FIG. 2 schematically illustrates one functional circuit block 21 of theprinted circuit board 10 of FIG. 1. For example, functional block 21 maycomprise the voltage control oscillator portion of the circuitry of atwo-way radio. In this example, the electrical components 11 may includea resistor, a capacitor, and an inductor, as well as other likeindividual components which are commonly used on a printed circuitboard.

FIG. 3 represents a basic embodiment 20 of the present invention.However, the various sizes and thicknesses shown in FIG. 3 are notproportional; rather, they are exaggerated to clarify the accompanyingdescription. In this embodiment 20, high density interconnect (HDI)technology is used to form a metal shield over a circuit, a circuitblock, or a portion of a circuit which is referred to hereinafter ascircuit 21. A circuit 21 is printed on a substrate 22. Circuit 21 andsubstrate 22 are covered by a layer of dielectric material 23. A layerof metal 27 is deposited over the dielectric layer 23. A plurality ofsolder pads 24 are formed on the dielectric layer 23 at predeterminedlocations relative to circuit 21. A small non-metallic area 26 surroundseach solder pad 24. Each solder pad 24 includes a microvia 25 extendingto the metal of circuit 21. One or more appropriate electricalcomponents 11 are soldered to the circuit 21 using the solder pads 24and the microvias 25 therethrough. In this manner, a metal shield isprovided over a circuit or circuits 21 where the emitted radiation isnot sensitive to the small non-metallic areas 26 surrounding the solderpads 24.

Referring still to FIG. 3, the substrate 22 is a typical insulatingsubstrate used with a printed circuit board. The insulating substrate 22can be of any material used in the electronics industry and is typicallya material such as glass-reinforced epoxy laminate, glass-reinforcedpolyimide laminate, flexible polyimide or polyester or ceramic. Circuit21 may be formed by any of the conventional methods known in the priorart, such as by photolithography and chemical etching. The layer ofdielectric material 23 can comprise a photoimageable material such asphotoresist, or other types of dielectric materials such as epoxy orpolylaminate and appropriately deposited or applied over the circuit 21and the substrate 22.

The metal layer 27 may be deposited by any conventional method such asphotolithography. The solder pads 24 each comprise a small area ofconductive material which allows for conventional soldering of aconnection point of circuit 21 to an electrical component 11. In thisembodiment, the solder pads 24 comprise the material from which themetal layer 27 is made and result by removing the metal of metal layer27 from the small areas 26 surrounding each solder pad 24. The metal oflayer 27 may be removed from areas 26 by, for example, chemical etching.The solder pads 24 are appropriately located above the circuit 21 wherethe electrical components are to be soldered.

The microvias 25 can be formed by any suitable high density interconnect(HDI) technology, including, but not limited to, photolithography, laserablation, or plasma etching, depending upon the material used for thedielectric 22. The microvias 25 penetrate through the solder pads 24 andthe dielectric material 22 to expose the metal of the circuit 21. Theelectrical components 11 are connected at the location of the solderpads 24 to circuit 21 by and conventional soldering method, such as byreflow soldering.

FIG. 4 schematically illustrates, the details of a solder pad 24 of FIG.3 formed by the metal layer 27 and the small non-metallic area 26. Themicrovia 25 is seen to be provided at the approximate center of solderpad 24 and through the layer of dielectric 23. The non-metallic area 26is not limited to any particular configuration; the only requirement isthat the solder pad 24 be electrically isolated from the layer of metal27. For example, the non-metallic areas 26 may be chemically etched fromthe metal layer 27 deposited by photolithography.

FIG. 5 illustrates an embodiment 30, shown in cross section, comprisingthe embodiment of FIG. 3 with the addition of the metal shield extendingdown and around the sides of the external periphery of circuit 21.

The side shielding added by the embodiment of FIG. 5 can be effectuatedby at least two ways. One method is to form a trench-like groove 31 indielectric layer 23 and around the periphery of circuit 21 and then fillor plate the groove 31 with metal simultaneously with the depositing ofmetal layer 27. Here, the groove 31 may be formed by conventionalphotolithographic methods. The side shield 32 formed by this method isshown on the left side of FIG. 5.

Another method of forming a metal shield along and around the sides ofcircuit 21 is to form a trench-like groove 33 in both the metal layer 27and the dielectric layer 23 and then filling or plating the groove 32with metal. Here, the groove may be formed by laser ablation. Theplating or filling of groove 32 can be accomplished by sputtering,electroless plating, ablative plating, or other like conventional means.The side shield 34 formed by this alternate method is shown on the rightside of FIG. 5.

In either of the alternative methods described above in the embodiment30 of FIG. 5, the metal shield 32 or 34 provided around the sides ofcircuit 21 makes electrical contact with the shielding provided by layer27 along the interface of the shields. However, small gaps arepermissible which can be used to provide electrical connections to andfrom the circuit 21. The top shield 27 and the connecting side shield 32or 34 need not follow the constraints of a conventionally formed metalshield; rather, the shielding provided by the present invention can beform fitted tightly around any desired circuit 21 however irregular itsperiphery. It is therefore to be noted that the trench-like grooves 31and 33 shown in FIG. 5 can be provided around a single circuit, aroundmultiple circuits, and even around individual portions of a circuit orcircuits, in order to insulate different functional blocks or circuitsfrom each other. By selectively locating the grooves and filling orplating the same with metal and by electrically connecting the metalshield 27 to the metalized periphery of the circuit or circuits, veryselective or complete top and side shielding can be effectuated. Smallgaps or apertures transverse to the longitudinal axis of the sideshielding can provide for circuit trace penetration and interfacingbetween and within the circuits. Indeed, the various combinations aresubstantially limitless and can be readily envisioned by one havingskill in the art.

FIG. 6 illustrates an embodiment 40 where the methods and apparatus ofthe present invention provide complete top, side and bottom shieldingaround, for example, a circuit block using HDI technology. Thisembodiment 40 is shown in FIG. 6, where it is seen that a metal layer 41is applied to a substrate 22. A first layer of dielectric 42 is appliedover the metal layer 41. The printed circuits and circuit blocks 21 aredeposited on the first layer of dielectric 42. A second dielectric layer43 is applied over the first dielectric layer 42 and over the circuit21. Trench-like grooves 44 are formed completely around the circuit orcircuits 21 which are desired to be completely shielded. The grooves 44penetrate through both layers of dielectric 42 and 43, exposing themetal in metal layer 41. A final layer of metal 45 is then depositedover dielectric layer 43 which simultaneously fills or plates thegrooves 44, thereby electrically connecting the ground planes of metallayers 41 and 45. This method of shielding is seen on the left side ofFIG. 6. As illustrated in FIG. 6, circuits 21, 21′ disposed between thefirst and second dielectric layers 42, 43 may be shielded individuallyor in groups in accordance with the present invention. For example, anadditional metal plated trench-like opening 44′ may be created asdescribed herein and inserted between individual circuits 21′ or groupsof circuits 21, 21′ to shield and isolate them from each other. In sucha case, the additional metal plated trench-like openings 44′ areelectrically connected to the other grooves 44 through metal layers 41and 27. The right side of FIG. 6 shows an alternative method ofshielding. Here, after each dielectric layer 42 and 43 is deposited, agroove 46 and 47, is made in each layer, respectively. Each groove 46and 47 is separately filled or plated with metal 48 and 49. Then themetal layer 45 is applied.

The substrate 22, dielectric layers 42 and 43, the metal layers 41 and45, the grooves 44, 46 and 47 and the metallization of the grooves 44,46 and 47 are formed or provided as explained above. The components 11are connected to the circuit 21 using the solder pads 24 and methodsdescribed above. Trace interfacing and penetrations are also provided asdescribed above.

Should it be desired, the embodiment 40 of FIG. 6 can include circuit orcircuit blocks 21 layered one on top of each other with a layer ofdielectric 50 therebetween as shown in FIG. 7. One or more microvias 51in the layer of dielectric 43 between the stacked circuits 21, which arethen filled with metal 52, can provide for electrical connections 53between the stacked circuits. It is to be noted that the above-describedmethod of isolating different functional blocks can be applied to theembodiment of FIG. 6 with the result that the different circuit blockscan each be completely surrounded on all sides or completelyencapsulated by a metal shield and thereby be shieldingly isolated fromeach other.

While the invention has been described, disclosed, illustrated and shownin certain terms or certain embodiments or modifications which it hasassumed in practice, the scope of the invention is not intended to benor should it be deemed to be limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A method for shielding one or more circuits of a printed circuitboard wherein said one or more circuits are printed on a substrate,comprising the steps of: depositing a layer of dielectric material overthe substrate and over the one or more printed circuits thereon;creating a trench-like opening in the dielectric layer such that thetrench-like opening surrounds the one or more circuits to be shielded;depositing a layer of metal over the layer of dielectric material andwithin the trench-like openings; creating a solder pad at each locationwhere an electrical connection is to be made to the one or more printedcircuits by removing a border of the metal layer surrounding eachconnection location, thereby leaving a portion of the metal layer withinthe border at the connection locations; and providing a microvia througheach solder pad penetrating the dielectric layer and terminating at themetal of the printed circuit.
 2. The method of claim 1, wherein the oneor more printed circuits comprise at least two printed circuits,including the step of providing a metal-plated trench-like openingbetween two printed circuits with the trench-like opening surroundingthe at least two printed circuits being electrically connected to themetal plated trench-like opening between the two printed circuits. 3.The method of claim 1, including the steps of providing one or more gapsin the metal deposited in the trench-like openings and routing a circuittrace through each of the one or more gaps.
 4. The method of claim 3,including the step of soldering an electrical component to one or moreof the solder pads.